CD44v9

CD44v9 is a splice‑variant isoform of the CD44 cell surface glycoprotein encoded by the CD44 gene on chromosome 11q13.2. The variant results from inclusion of exon 9 in the mRNA, adding a unique 11‑amino‑acid insert to the stem region of the protein. The extracellular domain of CD44v9 binds hyaluronic acid, osteopontin and other matrix components, while the cytoplasmic tail interacts with the cytoskeleton and signaling molecules such as ERM proteins.

The CD44 family mediates cell adhesion, migration, and signaling in normal tissues. CD44v9 is preferentially expressed in epithelial tissues undergoing rapid turnover, such as the skin, stomach, and mammary gland, and its expression is highly up‑regulated in several cancers, including gastric carcinoma, pancreatic ductal adenocarcinoma, colorectal cancer, and glioblastoma. In tumor cells, CD44v9 functions as a marker of cancer stem‑like cells and contributes to chemoresistance, epithelial‑mesenchymal transition, and metastasis. Mechanistically, CD44v9 stabilizes the cystine‑glutamate antiporter system Xc–, thereby enhancing glutathione synthesis and antioxidant capacity. This antioxidant role protects tumor cells from reactive oxygen species and apoptosis, contributing to tumor progression and therapy resistance.

In gastric cancer, increased CD44v9 correlates with poor overall survival and higher tumor grade. In pancreatic cancer, CD44v9-positive cells exhibit enhanced colony‑forming ability and tumor‑initiating capacity in xenograft models. Targeting CD44v9 with monoclonal antibodies or small‑molecule inhibitors has shown promise in preclinical studies, reducing tumor growth and sensitizing cells to chemotherapy.

Overall, CD44v9 serves as both a biological functional protein in normal physiology and a critical mediator of tumor aggressiveness, making it a potential diagnostic marker and therapeutic target in oncology.